When EN Publishing did the adventure path War of the Burning Sky in 2007, it was pretty linear with just a few NPCs who recurred and a few narrative throughline between adventures.
When we did ZEITGEIST: The Gears of Revolution in 2011, we upped our game a lot. I had a big notebook to keep track of all the linkages between adventures, the branching character arc possibilities of about 40 NPCs, and a suite of character hooks we encouraged GMs to use to weave the PCs into key moments of every single adventure.
When you play adventure 6 at 11th level, you go to a brand new country but can run into one NPC you met in your home city in adventure 2 when he fell afoul of a gang, another NPC you encountered while you were undercover on a train in adventure 4, a diplomat you protected in adventure 5 can help you get political connections in this new nation, AND a minor VIP who had cameos in adventures 1 and 3 is revealed to be a new villain trying to make a play for power.
Your interactions with them before affect what happens now, and your choices in this adventure might result in the villain being an ally later, and various other little flairs for the different NPCs.
And the adventures do a lot of work to help the GM keep track of all of this. But damn if it was not a fair bit of work for me as the line director. The original idea of having a different author for each adventure became infeasible after adventure 5 because I'd have to write 10,000 words myself just to bring a new guy up to speed, and they'd invariably not grasp all the nuances for what the long-term plan was.
But the groups who have finished the whole adventure path seem to have really appreciated how deep and rich the world ends up being, because we gave the players opportunities to build genuine relationships with recurring characters.